The most common and most likely answer to this crossword clue is the 7 letter word STENTOR
I've seen this crossword clue in The Guardian
S
T
E
N
T
O
R
More clues leading to the result STENTOR
Buddy explains!

The straight part
We know that "Loud speaker" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case loud speaker = stentor.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does STENTOR mean?
STENTOR noun- A person with a powerful or stentorian voice.
- Any protozoan of the genus Stentor.
- A part of the amplification system of a carillon.
- A howler monkey.